Gain control circuits



Sept. 19, 1944.

GAIN- G. W. FYLER GAIN CONTROL CIRCUITS Filed Oct.

GAI

FREQUENGY FREQU ENCY-- F IQ. 4. 70'

GAIN- FREQUENCY Inventor: George N. Fyler;

His Attorney.

Patented Sept. 19, 1944 GAIN CONTROL CIRCUITS George W. Fyler,Stratford, Conn, minor to General Electric Company, a corporation of NewYork Application October 21, 1941, Serial No. 415,919

9Claims.

My invention relates to gain control circuits for signal amplifiers andhas for one of its objects the provision of improved manual gain controlcircuits for high-frequency wide-band amplifying systems.

Another object of my invention is to provide improved means for varyingthe gain of a highfrequency amplifier without affecting the symmetry ofits band pass characteristic. v A further object of my invention is toprovide an improved manual gain control for a highfrequency thermionicamplifier wherein adjustment of a single movable contact membersimultaneously varies the potential on, one control electrode in orderto vary the gain of the device and also the potential on another controlelectrode in order to maintain the input capacitance of the devicesubstantially constant at all gain control settings.

The features of my invention which I believe to be novel are set forthwith particularity in the appended claims. My invention itself, however,together with further objects and advan-' tages thereof, may bestbeunderstood by reference to the following description taken in connectionwith the accompanying drawing, in which Fig. l is a circuit diagram of ahigh-frequency amplifier embodying my invention; Figs.

2, 3 and 4 are curves illustrative of certain operating characteristicsof the apparatus of Fig. 1; and Fig. 5 is a circuit diagram illustratinga modified form of my invention.

winding plus any other shunt cincuit capacities, all indicated by thedotted capacitor IS. The secondary winding I2 is similarly tuned by thedistributed capacity and other shunt capacities across the transformerwinding, represented by the dotted capacitor ii, in conjunction with theinput capacitance of the amplifier ll, represented by the dottedcapacitor l8 connected between the control grid l3 and cathode IS. Thecathode I3 is connected to ground through a resistor 20 shunted by abypass capacitor 2|. The capacitor 2| is of low impedance at the carrierand sideband frequencies so that the input capacitance l8 of the deviceIt is effectively connected directly in shunt to the secondary circuitI2. When a wide-band frequency characteristic is desired, either or bothof the circuits Ill and I2 may be shunted in a. conventional mannerbydamping resistors. Such a damping resistor 22 is shown connected inshunt to the secondary winding i2.

The anode 23 of the device It is connected to ground through a circuitincluding the primary winding 24 of an output coupling transformer 25, aresistor 26 and a suitable source of oper- Referring now to Fig. 1,high-frequency electrlcal waves are impressed upon the primary windingill of an input coupling transformer II from any suitable source, notshown. While my invention is not limited thereto, it has particularutility when these waves are of relatively high frequencies, of theorder of several megacycles ing device l4 represented as a pentode, andthe other terminal connected to ground by a conductor 15. The primaryand secondary windings l0 and I2 may be tuned to the mean operatingfrequency in any suitable manner. primary winding I0 is represented asbeing tuned by the distributed capacity of the transformer atingpotential represented conventionally by the battery 21. The resistor 26and potential source 21 are bypassed by a large capacitor 23. Thesecondary winding 29 of the transformer 25 is coupled to anysuitableutilization circuit or load device, not shown. This may,forexample, comprise another stage of amplification similar to thatrepresented in Fig. 1. The transformer 25 is tuned to the mean operatingfrequency in substantially the same manner as the trans former II, thetotal effective capacity across the primary winding 24 being representedby the dotted capacitor 30 and the total effective capacity across thesecondary winding 29 being represented by the dotted capacitor 31. Thesecondary winding 29 is also preferably shunted by a damping resistor 33to assist in obtaining a wide-band characteristic.

The upper terminal 34 of the resistor 26 is connected to the upperterminal 35 of the resistor 20 through a potentiometer resistor 36.

The screen grid 3'! of the pentode is connected to a tap 38 which isadjustable along the resistor 36. The tap 33 is preferably in the formof a sliding contact providing a smooth range of ad- Thus the justmentbetween the points 34 and 35. The screen grid 3'! is also bypassed toground for operating frequencies by the capacitor 33 and is connecteddirectly to the point 34 through a resistor 40.

The suppressor grid ll is internally connected istic at all settings.

to the cathode It in the type of pentode amplifier represented in. Fig.1.

It is known that the input capacitance of an electron dischargeamplifying device depends not only upon the tube .structure but alsoupon the operating potentials applied to the various electrodes. It hasheretofore been common practice to vary only the control grid bias inorder to change the over-all amplifier gain. This may providesatisfactory control in amplifying circuits operating at lowerfrequencies, where the input capacitance of the amplifieris negligibleas compared to externalcircuit capacities. However,- seriousdifilculties have been encountered in attempts to apply this form ofcontrol to highfrequency' amplifying circuits, such as the intermediatefrequency circuits of television apparatus, for example. The reason forthis apparently lies in the fact that the input capacitance between thecontrol grid and cathode of tubes available for this type of work maychange by as much as one or two micro-microfarads as the control gridbias is varied between its normal operating value and a valuecorresponding to anode current cutofl. As the control grid bias becomesmore negative the input capacitance decreases. In a high-frequency,wide-band amplifier of the type shown in Fig. 1 this effect is circuitin a sense to give increase relative low frequency response within thepass-band.

Fig. 2 illustrates typical response curves for a wide-band intermediatefrequency amplifier for three different values of control grid bias, theother operating potentials remaining fixed. Assume first-that theamplifieris adjusted to have a fiat frequency response curve at onevalue of operating bias, as represented by the curve 58. Then as thegrid bias is made more negative, the change in input capacitance rendersthe response curve unsymmetrical. Thus the dotted curve 6| is typical ofthe response obtained with one value of increased grid bias and thedotted curve 52 is representative of the response obtained with a stillhigher value of negative grid bias.

Conversely, it has been found that if the amplifier gain is varied bycontrolling the. screen grid potential, other operating potentialsremaining constant, then the opposite type of detuning occurs. Thus ifthe curve 60 in Fig. 3 is taken to represent the response curve for agiven value of screengrid potential, then the dotted curves 6| and 62illustrate the effect of progressively 'decreasing the screen gridpotential. It will be observed that these curves are almost exactlycomplementary to the curves 6! and 62 of Fig. 2,for certain potentialvalues.

In accordance with my invention these two effects just described arecombined in such a manner that a wide range of gain control may beeffected while yet retaining a fiat frequency response characteristic,It will be observed that the resistors 20, 26, 36, and 40 of Fig. 1 formthe component parts of a voltage divider connected directly acros thepotential source 21. It has been found that by a suitable choice ofthese resistances, with regard to the type of amplifier employed and theoperating voltages and frequencies, it is possible to obtain a widerange of gain control by moving the tap 38 while yet maintaining asubstantially symmetrical response character- Thus the curve 10 of Fig.4 is representative of operating conditions with the tap 88 connecteddirectly to the point 36, while tain particulars.

the curves 1| and 12 correspond to positions of the 16 tap l8progressively nearer the point 36. The ad- Just-ment and operation ofthe circuits, whereby these results are achieved, will now be explainedin greater detail.

First assume that the tap 38 is connected directly to the point 34. Thecontrol grid I8 is selfbiased by a predetermined value of anode andscreen grid current flowing through the resistor 20. This self-bias isalso assisted by a predetermined value of current flowing directlythrough the voltage divider resistors 26,. I6, and 20, the resistor 40being short-circuited in thi position of the tap 38. Consequently, thetotal efiective bia upon the control grid I3 is determined by the totalvoltage drop across the resistor 20 due to all these currents. Thepotential between the screen grid 31 and cathode i8 is equal to thevoltage drop across that section of the voltage divider between the tap38 and the point 86, i. e., across the entire resistor 36 in this case.

Assume now that the tap 88 is moved toward the point to some intermediatpoint such as that represented in Fig. 1. It will be apparent that theresistor 40 is now connected in parallel to the right-hand portion ofthe potentiometer resistor 36 so that the current drawn through thevoltage divider network is somewhat greater than before. In accordancewith the invention the resistors are so proportioned that although thescreen grid. potential now decreases, causing the anode and'fscreen gridcurrents through the amplifier l4 and resistor 20 to decrease,nevertheless the efiective bias potential between the control grid I3and cathode I8 is caused to increase by virtue of the increased currentdrawn through the voltage dividing-network from the potential source 21.In other words, the circuit constants are so selected that the voltagedivider current through the resistor 20 increases faster than the sum ofanode and screen grid currents decreases. The proper values for theseresistors are most easily determined by actual tests. Proper adjustmentis secured when the detuning effect resulting from the decrease inscreen grid potential is just compensated by a corresponding increase inthe net bias potential applied to the control grid iii. In actualpractice it has been found that most of the gain control action isaccomplished by variation of the screen grid potential and that only arelatively small variation in th control grid potential is required tomaintain the input capacity l8 substantially constant.

In the arrangement of Fig. 1 it will be observed that the screen gridpotential eventually decreases to zero as the tap 38 reaches the point35 and that the voltage divider current is a maximum for thisadjustment, since the resistor 40 is then connected in parallel with theentire resistor 36.

The modified form of my invention represented in Fig. 5 differs fromthat of Fig. 1 only in cer- Corresponding elements have been designatedby the same reference numerals and the function of these elements isessentially the same. Therefore they need not be repeated here. In thismodification a slightly difierent typ of pentode amplifier I8 i shownwherein the suppressor grid 4| is externally connected to the groundedtube shield. However, the principal difference resides in thearrangement of the several sections of the voltage dividing networkacross the potential source 21. The section of this network between thecathode and ground, comprising the resistor 20, is the same as in Fig.l, but the section between the screen grid 31 and cathode is consists ofa variable resistor 80, and the section between the screen grid 31 andthe positive terminal of the power supply source 21 consists of a fixedresistor 8|. A separate decoupling resistor 82 is provided for the anodecircuit of the amplifier in this embodiment of the invention, and asmall filter resistor 83 is included in the connection between screengrid 3'! and tap 38,

The operation of the amplifier shown in Fig. 5

' Letters Patent of the United States, is:

does not difier essentially from that of Fig. 1.

As in the case of Fig. 1, it will be observed that the total resistanceof the voltage divider network, measured across the three resistors 20,80 and BI, decreases when tap 38 is moved toward the left for thepurpose of reducing the screen grid potential and thereby the gain.Likewise, the circuit constants are so proportioned that the tendencyfor the input capacitance to increase as the screen grid is made lesspositive is automatically.

compensated by a simultaneou variation of the total current through theresistor which causes the control grid l3 to become slightly morenegatlve, and vice versa.

Any desired degree of compensation can readily be secured by properlyproportioning the various circuit elements. It will also be appreciatedby those skilled in the art that other equivalent forms of voltagedivider networks maybe employed to provide the desired action.

For completeness of illustration only, and not in any sense by way oflimitation, the following circuit constants are given as being typicalfor a high-frequency wide-band amplifier of the type represented in Fig.5. These constants have been found to be satisfactory for the circuitelements of a signal amplifier adapted to operate over a frequency bandof 42-48 megacycles. In this particular apparatus the pentode amplifierH was a type 7V7. Other circuit constants were as follows:

Resistor 20 ohms 4'7 Capacitor 2| mmf 3900 Capacitor 28 mmf 3900Capacitor 39 mmf 3900 Resistor 80 ohms (variable) 20,000 Resistor 8|ohms 15,000 Resistor 82 do 470 Resistor 83 do 470 tials vary in themanner of my invention. 1

eliminate the need for any such mechanical complications by utilizingonly a single movable contact member.

the potentials on the two difierent electrodes to vary in unison and toremain in proper ratio over a wide range of volume control adjustments.

While I have shown particular embodiments of my invention, it will ofcourse be understood that I do not wish to be limited thereto sincevarious modifications may be made, nd I contemplate by the appendedclaims to cover any such modifications as fall within the true spiritand scope of my invention.

This not only assures a quieter volume control action but automaticallycauses 1. In a system for amplifying currents having frequenciesextending over a wide band, an amplifier including a pair of controlelectrodes, one of said electrodes being a current carrying electrode, apotential source, means for impressing operating potential on said oneof said electrodes, said means comprising a connection from said oneelectrode to a variable point on said-source, means for impressingoperating potential on the other of said electrodes, the gain of saidamplifier being dependent upon both said potentials, means for varyingthe position of said point to vary the potential on said one electrode,and entirely electrical means responsive to variation in the position ofsaid point for varying the potential on said other electrode in the samegain-varying sense, the potentials of said electrodes being varied insuch amounts as to maintain the input capacitance of said amplifierconstant.

2. In a system for amplifying currents having frequencies extending overa wide band, an amplifier including a pair of control electrodes, apotential source, means for impressing operating potential on one ofsaid electrodes, said means comprising a connection from said oneelectrode to a variable point on said source, means for impressingoperating potential on the other of said electrodes, the gain of saidamplifier being dependent upon both said potentials and the frequencyresponse characteristics of said amplifier being oppositely afiected bypotential variations on the respective electrodes which produce changesof gain in the same sense, means for varying the positionof said pointto vary the potential on said one electrode, and entirely electricalmeans responsive to variation in the position of said point for varyingthe potential on said other electrode in the same gain-varying sense andin such .relationship as to maintain the frequency responsecharacteristic substantially constant.

3. In a high-frequency wide-band amplifying system, an amplifier havinga cathode,.a pair of control electrodes and an anode, one of saidelectrodes being a current carrying electrode, input and output circuitsoperatively associated with said device, a source of current, meansresponsive to said current for impressing an operating potential on saidone of said control electrodes, the gain of said amplifier beingdependent upon said potential andalso upon the potential of said othercontrol electrode, the input capacitance of said amplifier beingoppositely afiected by potential variations on the respective electrodeswhich produce changes of gain in the same sense, means for varying saidcurrent to vary the potential on said one electrode and the amplifiergain, and electrical means responsive to said current variations forvarying the potential on said other electrode in the same gain-varyingsense and in an amount suflicient to maintain said input capacitancesubstantially constant.

4. In a high-frequency amplifying system, an electron discharge deviceincluding a cathode, an anode, an input electrode and a current carryingelectrode, input and output circuits operatively associated with saiddevice, means responsive to discharge current flowing in said device forim pressing a self-biaspotential on said input electrode, said meanscomprising an impedance com mon to said circuits, means for causing anadditional current to flow through said impedance in a direction toassist said self-bias potential,

means for impressing operating potential between said current carryingelectrode and cathode, and means for simultaneously varying saidoperating potential and said additional current in a predeterminedrelationship such that the net bias on said input electrode becomes morenegative as said operating potential is decreased, and vice versa.

5. In a high-frequency amplifier including an electron discharge devicehaving a cathode, a signal input electrode, a positive gain-controlelectrode and an output electrode, means responsive to current flowingin said device for impressing a negative bias voltage on said inputelectrode, said means comprising an impedance common to the input andoutput circuits of said device, means comprising a source of potentialfor passing an additional current through said impedance to provideadditional negative bias voltage for said input electrode, means forimpressing a positive potential on said gain-control electrode, meansfor varying said positive potential to adjust the gain of said device,whereby the input capacitance of said device tends to vary, and meansfor simultaneously varying said additional current in the opposite senseand in an amount sufiicient to produce a change in the net bias voltageon said input electrode opposing said tendency.

6. In a high-frequency signal amplifying system including a thermionicamplifier provided with at least a cathode, a control grid, a positivegrid and an anode, means comprising a fixed resistor bypassed for signalfrequencies and common to the control grid and anode circuits forimpressing a self-bias on said control grid in response to current insaid device, a source of potential, a voltage divider connected acrosssaid source, said divider having a plurality of resist- ,ance sections,one of said sections comprising said resistor and being connected insuch polarity that current through said divider assists said self-bias,means for impressing an operating potential between said positivegrid-and cathode in response to the voltage drop produced by saidcurrent in another section, means for varying the total resistance ofsaid divider comprising means for varying the resistance of at leastsaid other section, the resistance values of the sections of saiddivider being interrelated to produce an increase in the net controlgrid bias as said positive grid potential decreases and vice versa.

7. In combination with a high-frequency wideband signal amplifierincluding a cathode, control grid, screen grid and anode, a potentialsource having positive and negative terminals, a voltage dividingresistance network connected across said terminals and drawing apredetermined current, said network having a first section between saidpositive terminal and screen grid carrying screen current and saidpredetermined current, a second section between said screen grid andcathode carrying said predetermined current and a third section of fixedresistance between said cathode and negative terminal carrying saidscreen current, said predetermined current and the anode current of saiddevice, means for biasing said control grid in response to the currentsin said third section, means for varying the resistance of said secondsection, whereby the gain and input capacitance of said device arevaried, and means for simultaneously varying the series resistance ofsaid first and second sections in the same sense as said second sectionand in such ratio thereto that said predetermined current varies at afaster rate than the sum of said screen grid and anode currents, wherebyan opposite and compensatory change in input capacitance is produced byvariation of said control grid bias.

8. In a high-frequency signal amplifying system, a thermionic devicehaving a cathode, a control grid, a screen grid and an anode, a fixedbias resistance connected between said cathode and a reference point,input and output circuits respectively connected from said control gridand from said anode to said point, a source of screen grid potentialhaving a negative terminal connected to said point and a positiveterminal connected to said cathode through a second resistance, a thirdresistance, connected between said screen grid and a connection point onsaid second resistance positive with respect to said cathode, aconnection from said screen grid to a tap on that portion of said secondresistance between said connection point and cathode, means foradjusting the position of said tap on said portion to vary the screengrid potential and gain 'of said device, whereby the input capacitanceof said device tends to change, said resistances being proportioned tomaintain the control grid bias automatically related to said screen gridpotential for all adjustments of said tap in such ratio that changes ininput capacitance are substantially prevented.

9. In a high-frequency signal amplifying system, a thermionic devicehaving a cathode, a control grid, a screen grid and an anode, a fixedbias resistance connected between said cathode and a reference point,input and output circuits respectively connected from said control gridand from said anode to said point, a source of screen grid potentialhaving a. negative terminal connected to said point and a positiveterminal connected to said screen grid through a second resistance, avariable resistance connected between said screen grid and cathode,means for adjusting said variable resistance, thereby to vary the screengrid potential and the gain of said device,

whereby the input capacitance of said device tends to change, saidresistances being proportioned to maintain the control grid biasautomatically related to said screen grid potential for all adjustmentsof said variable resistance in such ratio that said changes in inputcapacitance are substantially prevented.

GEORGE W. FYLER.

